1. Field of the Invention
The invention relates to a composite material that is designed in particular as a substrate glass for light emitting diodes (“LED's”), and a method for the production thereof.
2. Description of Related Art
Organic LED's (usually called OLED's) have been continuously further developed improved in recent years, which addresses efficiency and service life. Nevertheless there is a great need for improving such LED's, especially to be able to make products available that are competitive with conventional LED technology.
Usually, only about 20 to 25% of the light produced in the emitter layer is emitted by LED's. A large fraction of the light generated remains in guided optical modes within the organic layers or the substrate. A portion of this loss of light can be attributed to total reflections at the substrate/air interface.
Possibilities are known from the state of the art for reducing these losses of light.
A simple method for increasing the light decoupling is to apply a layer that diffusely scatters the light and thus reduces the fraction of light that is lost by total reflection. It is known how to introduce scattering particles, for example, into a light-transmitting substrate. Such light-scattering layers with a transmitting matrix are described, for example, in JP 2001 356207.
These approaches to a solution are based on the scattering properties of the dispersed particles introduced, which have a different index of refraction than the adjacent medium. Because of the deflection at the phase boundaries, fewer photons remain captured in the layer.
It is also known how to introduce foreign bodies, especially microparticles or nanoparticles, into an organic electroluminescent material.
The document WO 03/061028 A2 shows a light-emitting structural element in the form of an OLED in which microparticles are embedded in a polymer layer in order to increase the light decoupling coefficient.
Another approach to a solution is to introduce a color conversion layer that functions as a filter and is formed based on a resin transparent to light into which a light-emitting material is introduced, which, on the one hand, absorbs the light of the OLED and, on the other hand, emits fluorescent light in the visible region. The drawback here is that because of the polarizing mechanism of action, the efficiency is reduced by the previously described losses of light.
The document DE 102004035965 shows an OLED with a scattering cover that has inorganic particles or a structured interface. A polymer is used as the embedding material for the inorganic scattering particles.
An important drawback of such layered systems based on organic polymers is the low thermal and mechanical resistance, which limits subsequent processing steps, and in many cases the lack of long-term stability.
Also, the polymers used usually have an index of refraction below or equal to 1.5. For this reason, layers based on organic polymers usually lead to high losses of light because of total reflections at the substrate/polymer layer interface.